Many computers are connected to publicly-accessible networks such as the Internet. This connection has made it possible to launch large-scale attacks of various kinds against computers connected to the Internet. A large-scale attack is an attack that involves several sources and destinations, and which often (but not necessarily) involves a large traffic footprint. Examples of such large-scale attacks may include:                viruses, in which a specified program is caused to run on the computer, which then attempts to spread itself to other computers known to the host computer (e.g., those listed in the address book),        denial of service attacks (DoS), in which a group of computers is exposed to so many requests that it effectively loses the ability to respond to legitimate requests. Many viruses and worms indirectly cause DoS attacks as well for networks by sending a huge amount of traffic while replicating. Distributed denial of service (DDOS) occurs when an attacker uses a group of machines (sometimes known as zombies) to launch a DoS attack. Backdoor or Vulnerability Scanning: Another form of large-scale attack is where an intruder scans for backdoors at machines or routers. A backdoor is a method by which a previously attacked machine can then be enlisted by future attackers to be part of future attacks.        
Large-scale span: Spam is unsolicited network messages often sent for commercial purposes. Large-scale spam is often simply the same as (or small variants of) the spam sent to multiple recipients. Note that this definition of spam includes both email as well as newer spam variants such as Spam Sent Over Instant Messenger.
A specific form of attack is an exploit, which is a technique for attacking a computer, which then causes the intruder to take control of the target computer, and run the intruder's code on the attack machine. A worm is a large-scale attack formed by an exploit along with propagation code. Worms can be highly efficacious, since they can allow the number of infected computers to increase geometrically.
Many current worms propagate via random probing. In the context of the Internet, each of the number of different computers has an IP address, which is a 32-bit address. The probing can simply randomly probe different combinations of 32-bit addresses, looking for machines that are susceptible to the particular worm. Once the machine is infected, that machine starts running the worm code, and again begins the Internet. This geometrically progresses. However, future worms may not use random probing, so probing can only be used as one sign of a worm.
The worm can do some specific damage, or alternatively can simply take up network bandwidth and computation, or can harvest e-mail addresses or take any other desired action.
A very common exploit is a so-called buffer overflow. In computers, different areas of memory are used to store various pieces of information. One area in memory may be associated with storing information received from the network: such areas are often called buffers. However, an adjoining area in the memory may be associated with an entirely different function. For example, a document name used for accessing Internet content (e.g., a URL) may be stored into a URL buffer. However, this URL buffer may be directly adjacent to protected memory used for program access. In a buffer overflow exploit, the attacker sends a URL that is longer than the longest possible URL that can be stored in the receiver buffer and so overflows the URL which allows the attacker to store the latter portion of its false URL into protected memory. By carefully crafting an extra long URL (or other message field), the attack or can overwrite the return address, and cause execution of specified code by pointing the return address to the newly installed code. This causes the computer to transfer control to what is now the attacker code, which executes the attacker code.
The above has described one specific exploit (and hence worm) exploiting the buffer overflow. A security patch that is intended for that exact exploit can counteract any worm of this type. However, the operating system code is so complicated that literally every time one security hole is plugged, another is noticed. Further, it often takes days for a patch to be sent by the vendor; worse, because many patches are unreliable and end users may be careless in not applying patches, it may be days, if not months, before a patch is applied. This allows a large window of vulnerability during which a large number of machines are susceptible to the corresponding exploit. Many worms have exploited this window of vulnerability.
A signature is a string of bits in a communication packet that characterize a specific attack. For example, an attempt to execute the perl program at an attacked machine is often signalled by the string “perl.exe” in a message/packet sent by the attacker. Thus a signature-based blocker could remove such traffic by looking for the string “perl.exe” anywhere in the content of a message. The signature could, in general, include header patterns as well as exact bit strings, as well as bit patterns (often called regular expressions) which allow more general matches than exact matches.
While the exact definition of the different terms above may be a matter of debate, the basic premise of these, and other attacks, is the sending of undesired information to a publicly accessible computer, connected to a publicly accessible network, such as the internet.
Different ways are known to handle such attacks. One such technique involves using the signature, and looking for that signature in Internet traffic to block anything that matches that signature. A limitation of this technique has come from the way that such signatures are found. The signature is often not known until the first attacks are underway, at which point it is often too late to effectively stop the initial (sometimes called zero-day) attacks.
An Intrusion Detection System (IDS) may analyze network traffic patterns to attempt to detect attacks. Typically, IDS systems focus on known attack signatures. Such intrusion detection systems, for example, may be very effective against so-called script kiddies who download known scripts an attempt to use them over again at some later time.
Existing solutions to attacks each have their own limitations. Hand patching is when security patches from the operating system vendor are manually installed. This is often too slow (takes days to be distributed). It also requires large amounts of resources, e.g., the person who must install the patches.
A firewall may be positioned at the entrance to a network, and review the packets coming from the public portion of the network. Some firewalls only look at the packet headers; for example, a firewall can route e-mail that is directed to port 25 to a corporate e-mail gateway. The firewalls may be useful, but are less helpful against disguised packets, e.g., those disguised by being sent to other well-known services.
Intrusion detection and prevention systems, and signature based intrusion systems look for an intrusion in the network. These are often too slow (because of the time required for humans to generate a signature) to be of use in a rapidly spreading new attack.
Other systems can look for other suspicious behavior, but may not have sufficient context to realize that certain behavior accompanying a new attack is actually suspicious. For example, a common technique is to look for scanning behavior but this is ineffective against worms and viruses that do not scan. This leads to so-called false negatives where more sophisticated attacks (increasingly common) are missed.
Scanning makes use of the realization that an enterprise network may be assigned a range of IP addresses, and may only use a relatively small portion of this range for the workstations and routers in the network. Any outside attempts to connect to stations within the unused range may be assumed to be suspicious. When multiple attempts are made to access stations within this address space, they may increase the level of suspicion and make it more likely that a scan is taking place.
This technique has been classically used, as part of the so-called network telescope approach.